In communication systems which use non-geostationary satellites, the number and orientation of satellites in view of a ground-based terminal varies during a call. Thus, the communication link between the terminal and any one satellite may become weaker as the elevation angle of the satellite decreases and ultimately the link may become inoperable as the satellite moves out of sight of the terminal. It is therefore desirable to select another satellite for communication with the terminal, in a procedure known as "handover". The document U.S. Pat. No. 3,349,398 describes one such method. However, handover between satellites may result in loss of part of the signal, or sudden variations in signal quality, which are unacceptable in voice or data communications.
Furthermore, the line of sight between the terminal and a particular satellite may become obstructed by buildings, trees or other obstacles as the terminal or the satellite moves during a call. This effect is known as "blockage", and leads to fading in the received signal.
Signal fading may also occur when a signal transmitted by a satellite is reflected off the ground or buildings and the reflected signal is received at the terminal together with the direct signal. The phase difference between the direct and reflected signals may lead to destructive interference at the terminal, so that the received signal strength is reduced. This is known as "multipath" fading.
The document WO-A-93 09578 discloses a satellite communication system in which the satellites monitor the quality of signal received from a terminal and determine which one is best suited to handle the call to the terminal. One of the satellites re-transmits the signal received from the terminal to other satellites or gateways.
The conference paper "The Globalstar Mobile Satellite System for Worldwide Personal Communications" by Wiedeman and Viterbi, 3.sup.rd International Mobile Satellite Conference, Jun. 16-18 1993, Pasadena, Calif. discloses a communication system in which return link signals are received by two or three satellites; gateway stations measure the signal level of each of these alternate paths and control which signal paths are used. This system is exclusively designed for use with code-divided multiple access (CDMA).
However, CDMA suffers from a number of drawbacks when used for mobile communications. The mobile terminals are complex, since they require a separate decoder for each satellite path. Moreover, CDMA is inefficient in frequency re-use unless the users are evenly distributed, and power levels cannot be freely varied for each user without causing interference for other users. Furthermore significant interference takes place at peak levels of use.